build: use pkg-config to detect libmodplug

Fix static build, at least for OSX.

configure

@@ -4806,7 +4806,7 @@ enabled libgsm            && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
                                    check_lib "${gsm_hdr}" gsm_create -lgsm && break;
                                done || die "ERROR: libgsm not found"; }
 enabled libilbc           && require libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc
-enabled libmodplug        && require libmodplug libmodplug/modplug.h ModPlug_Load -lmodplug
+enabled libmodplug        && require_pkg_config libmodplug libmodplug/modplug.h ModPlug_Load
 enabled libmp3lame        && require "libmp3lame >= 3.98.3" lame/lame.h lame_set_VBR_quality -lmp3lame
 enabled libnut            && require libnut libnut.h nut_demuxer_init -lnut
 enabled libopencore_amrnb && require libopencore_amrnb opencore-amrnb/interf_dec.h Decoder_Interface_init -lopencore-amrnb